if you wanted to use a 31 spline differential for a late model cougar independent rear end, could you:

-switch your axles to 31 spline by swapping the inner 28 spline cv joints with the 31 spline inner joints of a late model cobra (2003 mustang cobra for example)?

-swap your 28 spline halfshafts with the 31 spline (and beefier) 2003 cobra halfshafts. even though the cobra shafts are shorter (from what i've read on forum posts), if the difference in length is minimal, could you just have a couple of steel spacers made to fit in between the inside of the spindle/bearing assembly and the outer cv joint of the halfshaft?

I wonder what the irs Explorer inner cv joints are like?? Or even the whole axle? Much more common and therefore cheaper from the salvage. Maybe another way to get a 31 spline carrier in?

From what I have read the outer joint is the weak spot anyway. The 28 spline carrier will wallow the cross shaft in time, so the only real upgrade here would be the hd carrier with a larger cross shaft.

Last week I snapped my 28 tooth axle stub in my diff. When I measured the shaft diameter of the 28T. it was 1.020" I also have a complete Explorer diff assy. here and a 31T. axle stub for it. The diameter of the 31T is 1.297" so the cross section of the axle is 61% larger. Also the stub doesn't neck down from the spline OD like it does on the 28T stub. I compared parts and the trunion ends are now huge and a larger seal, the bearings the same size on both. I can measure them and post the size but they do have an extra step on them.
My new car under construction has a complete Mk VIII suspension assembly under it and I'm now leaning on installing the 31T Explorer diff in it for reliability.
With the anticipated power output, I know the aluminum 28T case will not hold up to the rigors of road racing so the 31T will go in. Axle buildup will be the only problem for the installation. Maybe if I have some complete Explorer axles I can have custom shafts splined to fit between the ends. This way I'd use the Explorer trunion (inner) ends and the Mk VIII outer ends.

Turns out the outer cv joint has 31 splines like the inner. The actual shaft is as large or larger than a Cobra[just eyeballed it]. There is no way the hub would work on a t-bird. I would have measured the actual shaft length, but this is not anything worth pursuing to me. Too much machine work involved to make it work.

I wouldn't mind being able to use a 31 spline carrier only because the cross shaft looks twice as strong.

I've twisted the splines on my Mark and had to replace them less than a year ago. Those were the stock ones wtih 115K on them. I haven't checked the ones on the Tbird lately but those are R Axle shafts.